In the electrical discharge machining (EDM) process, a tool electrode in the form of a three-dimensionally shaped solid body or piece of sheet metal is juxtaposed with a workpiece electrode across a machining gap filled with a liquid machining medium or dielectric (e.g. kerosene, transformer oil, distilled water or weakly conductive water). Electric energy in the form of discrete electrical pulses is supplied across the machining gap to effect a succession of electrical discharges between the tool and workpiece electrodes to remove material from the latter. As material removal proceeds, the tool electrode is advanced relatively towards the workpiece electrode by a servo system adapted to maintain the machining gap substantially constant in size, thereby permitting material-removal discharges to be successively created and the tool electrode to be progressively sunk into the workpiece electrode so that a cavity generally conforming in shape to the tool electrode and of a desired depth may eventually be formed in the workpiece electrode. In such sinking type EDM operations, the tool electrode may be advanced vertically by displacing a supporting machine-head spindle with the servo system motor and additional relative movements may be imparted to the tool electrode and the workpiece electrode in a horizontal plane orthogonal to the spindle axis so as to generate in the workpiece a cavity similar in shape to the tool electrode or different therefrom.
Parameters of individual and successive electrical discharges, i.e. pulse on-time (.tau.on), peak current (Ip) and off-time (.tau.off) are determinative of machining results, e.g. removal rate, surface roughness and relative electrode wear and, therefore, are individually or in combination particularly adjusted to establish a particular machining condition suitable for achieving the desired machining end result factors.
In the course of the machining operation, it is important to ensure that chips, gases and tar produced by machining discharges are removed from the gap site. To this end, means for continuously or intermittently flushing the machining gap with a fresh machining liquid has commonly been utilized and also means for intermittently retracting the tool electrode away from the workpiece electrode or continuously or cyclically reciprocating the tool electrode away from and towards the workpiece electrode has been provided to allow the fresh machining medium to be pumped into the machining gap and the machining contaminants to be forcibly carried away from the latter.
In order to increase the removal rate, it is also desirable to operate the electrode-feed servo-control system so as to minimize production of non-striking pulses, i.e. pulses which do not cause discharge. The gap spacing can accordingly be reduced but this can also facilitate production of a continuous arc or result in difficulty in gap flushing or decontamination. The attempt to facilitate machining discharges tends to lower the removal rate while the setting of the servo system so as to widen the threshold gap spacing leads to an increased frequency of non-striking pulses.
It will be apparent that a reliable and accurate mechanical arrangement is essential to achieve desired movements of the tool or movable electrode and that this arrangement should also desirably operate in conjunction with the machining liquid flushing unit. In conventional arrangements, a common motor has typically been used to perform both the servo and electrode reciprocating operations; the motor has been coupled drivingly with the machine-head spindle and operated in response to signals furnished from two independent sources. In these arrangements, the tool electrode must be reciprocating driven so as to be integral with the machine-head spindle; it has been found that, especially when it is moved down in each reciprocating cycle at a high speed, it tends to over-travel and tends to collide with the workpiece electrode, thus damaging the tool or workpiece electrode or both. There has accordingly been a practical limit both in increasing the stroke and reducing the period of reciprocation. Another problem with the conventional arrangements is that it has been difficult to establish the stroke and the period of electrode reciprocation independently from one another and over wider ranges which are required in accordance with the particular surface areas and shapes of the tool electrode, particular machining depths and particular EDM conditions encountered by the machining gap. Unduly costly and highly complex sophistication of the equipment is unavoidable in earlier systems to achieve the desired results.